I am pretty sure you'll have little problem hiding most of the engine inside the Decathlon. I just finished a Sportsman Aviation Super Decathlon 86" wing with a Homelite 30cc 2stroke hidden completely within the cowl, except the B&B muffler stinger tips and the sparkplug cap. The Ryobi genIII 26cc has a fairly large aluminum (or Magnesium?) crankcase, which doubles as the oil sump, but I still think you'll be allright.

I am not so sure about J-3 Cubs. I have two WM 1/4 J-3 Cub ARFs, and their cowls are much narrower.